1. Field of the Invention:
The present invention concerns electronic signal processing and, more particularly, the field of pulse-echo ultrasound imaging systems having speckle noise interference.
2. Discussion of Background:
Medical technology is replete with applications of ultrasonic imaging and particularly in the use of pulse-echo B-mode tomography. Using this mode of ultrasonic imaging, acoustic echoes which return to a transducer are displayed as brightness levels which are proportional to the echo amplitude. Cross sectional images of the object are formed by displaying the brightness levels in proportion to the echo range and the position of the transducer.
In order to form the tomographic image, a fixed focus is formed by a plurality of piston-like piezoelectric transducers which are mechanically scanned. An alternative to this arrangement is now being used in many medical ultrasound imaging devices. This alternative relies on segmented array Ultrasonics, Academic Press: London and New York, 1977.
Another type of segmented array transducer is the sectored linear phased array consisting of a single group of transducer elements which is focused and steered over a sector angle during the transmission and reception mode by properly timing the transmission signals and the receive mode echoes. An example of the sectored linear phased array is given in the article entitled "Beam Steering with Linear Arrays" by 0. T. von Ramm and S. W. Smith, IEEE Transactions on Biomedical Engineering, BME-30, 438-458, 1983.
Pulse-echo imaging devices utilize radiation which are primarily characterized as coherent radiation. This exists despite the fact that there are significant band widths associated with the short bursts of ultrasonic energy emitted by the transducer. Because coherent radiation is used, an undesirable interference pattern or "speckle" noise occurs which is superimposed on the ultrasonic images and this "speckle" significantly degrades the image quality. As an analogy, ultrasonic speckle compares favorably with laser speckle in the sense that speckle is not present in any image which is formed by incoherent radiation. This is true whether the incoherent radiation is ultrasound or light.
The recognition and its analogy to laser speckle is described in "Speckle in Ultrasound B-mode Scans", by C. B. Burckhardt, IEEE Transactions on Sonics and Ultrasonics Volume SU-25, pages 1-6, January, 1978. Aside from discussing the existence of speckle, Burckhardt also derived the first order statistics of speckle and described how to reduce speckle contrast in an ultrasound image through the technique of spatial compounding. Accordingly, the average of N uncorrelated samples of an object volume from several independent transducer orientations in both the transmit and the receive modes reduces the contrast of the speckle noise (i.e. increases the speckle signal to noise ratio, SNR) by N.sup.1/2. This technique of reducing ultrasound image speckle uses spatial compounding in which the object volume is interrogated from more than one direction producing the independent samples of each target. The various schemes have been proposed with respect to spatial compounding and nearly all of these systems incorporate linear phased array transducers producing an image of either a sector or a rectangular format. Examples of these proposed systems exist as follows:
Carpenter, D. A., Dadd, M. J., and Kossoff, G., A multimode real time scanner, Ultrasound Med. and Biol. 6, 279-284 (1980). PA0 Berson, M., Roncin, A., and Porcelot, L., Compound scanning with an electrically steered beam, Ultrasonic Imaging 3, 303-308 (1981). PA0 D. P. Shattuck and O. T. von Ramm, "Compound scanning with a phased array," Ultrasonic Imaging, 4, 93-107 (1982). PA0 F. L. Thurstone, O. T. von Ramm, J. G. Abbott, H. B. Butts, and D. P. Shattuck "On Producing Compound B-scans At High Image Frame Rates" Proceedings Annual Meeting, Amer. Inst of Ultra in Med. San Diego, CA, Oct. 19-23, 1978, p.54. PA0 D. P. Shattuck, O. T. von Ramm, M. D. Weinshenker "Increased Data Acquisition Rate Through Parallel Processing," Ultrasonic Imaging 4, 196, 1982. PA0 O. T. von Ramm, and D. P. Shattuck, "Explososcan: A parallel Processed Phased Array Scanner," Ultrasound in Medicine and Biology, Vol 8, (Supp 1), p 202, 1982. PA0 D. P. Shattuck, "Improved Ultrasonic Imager Utilizing Spatial Compounding and Parallel Processing," Ph.D. Dissertation, Duke University, Durhan, NC 27706, 1982. PA0 D. P. Shattuck, M. D. Weinshenker, S. W. Smith, and O. T. von Ramm, "Explososcan: A Parallel Processing Technique for High Speed Ultrasound Imaging with Linear Phased Arrays," J. Acoust. Soc. Amer. 75, 4, 1272-1282, 1984.
Although only slight attention has been given to spatial compounding using piston transducers or annular arrays in the form of mechanically scanned circular transducers, some success has been achieved through the utilization of a mechanically steered spatial compound system using four adjacent servo controlled piston transducers as discussed by T. Matzuk and M. L. Skolnick "Real time Compound Scanner Using Four Servo Controlled Transducers" Proceedings Annual Meeting, Amer. Inst. of Ultra in Med. Montreal, Aug. 27-31, 184, 1979.
Furthermore, significant recent speckle reduction has been achieved by signal processing techniques which are similar to spatial compounding such as illustrated in FIG. 1 which shows a piston transducer 10 cut into eight pie-shaped segments. In this design, the receive mode echo signals S.sub.i from each segment is envelope detected and then summed to produce a processed signal, X=.SIGMA..sub.i=1.sup.8 Det.lambda.S.sub.i ], for a compound image. The Det[S.sub.i ]refers to the envelope detected RF signal. This is illustrated by the design of M. S. Patterson and F. S. Foster, "Improvement and quantitative assessment of B-mode images produced by an annular array/cone hybrid", Ultrasonic Imaging, 5, 195-213, 1983. A single orientation of the transmit aperture is used and the receive aperture is divided into several sub-apertures to obtain independent samples of the target. A hybrid transducer was used consisting of a spherically shaped, focused transducer combined with a concentric planar transducer disk and two aluminum mirrors. A full circular aperture was used in the transmit mode with either the spherical transducer or a planar disk. In a receive mode, the signal outputs from the eight sectors of either the spherical transducer or a planar disk were combined using multiplicative processing or summation. The techniques of the FIG. 1 illustrate, for example, "phase insensitive sector addition" when referring to the above equation with respect to the processed signal. The Det[S.sub.i ]represents the envelope detected RF signal. The value x is the sum of the signals for the eight sectors of the receive mode transducer.
This type of system suffered approximately a three-fold loss of lateral resolution while achieving some speckle contrast reduction. A similar proposal using X=.SIGMA..sub.i=1.sup.6 Det[S.sub.i +S.sub.i+2 ] was accomplished by M. S. Patterson, "The application of axicon transducers to medical ultrasound imaging", Ph.D. Thesis, University of Toronto, 1983. A third technique involved X=.SIGMA..sup.8.sub.i=1 [S.sub.i ].sup.1/8. This is referred to as the multiplicative processing. Lastly, a technique wherein X={.pi..sup.8.sub.i=1 [Det(S.sub.i)]}.sup.l/8. This technique, which is referred to as the "phase insensitive sector multiplication", also yielded speckle reduction, at the expense however of loss in the lateral resolution. This technique is disclosed by the article of A. T. Kerr, M. S. Patterson, F. S. Foster, and J. W. Hunt, entitled "Speckle Reduction in Pulse Echo Imaging Using Phase Insensitive and Phase Sensitive Signal Processing Techniques", Ultrasonic Imaging 8, 11-28 (1986). Needless to say there have been many attempts to reduce optical speckle by reducing the coherence of laser imaging systems as detailed by T. S. McKechnie, "Speckle Reduction", in Laser Speckle and Related Phenomena, J. C. Dainty, ed., Springer-Verlag, N.Y., 123-170, 1975.
Aside from the systems which attempt to reduce speckle by reducing the coherence of the laser imaging system, each of the prior art signal processing techniques concerning spatial compounding with linear arrays require an evaluation of the tradeoff of increased speckle SNR versus loss of lateral resolution. For a given imaging task in the body, it must be determined whether it is better to use the full transducer aperture with optimum lateral resolution in a simple scan or to divide the aperture into N subapertures to achieve N independent samples of the speckle noise and increase the speckle SNR by (N).sup.1/2 while losing some lateral resolution. Previous investigations have not confirmed the clinical effectiveness of speckle reduction via spatial compounding, but instead have relied on various studies of phantoms to evaluate the success of previous speckle reduction techniques. These studies include such things as the point spread response of the imaging system, the ratio of mean to standard deviation, i.e. the signal to noise ratio of the speckle probability density function, the cross-correlation coefficients between samples of the target volume and measurements of the detectability of lesions in tissue mimicking materials such as contrast detail diagrams and the area-wide signal to noise ratio or the contrast to speckle ratio. Each of these studies have revealed that there are consistent losses of lateral resolution or image contrast as measured by the point spread response in exchange for the improvement in the speckle SNR.
As indicated previously, there is a similarity between the speckle reduction in ultrasound images and the analog with respect to speckle reduction of optical imaging systems. A method which is similar to the above-discussed methods with respect to ultrasound has been set forth with respect to optical imaging as indicated by the above-referred to article by McKechnie. In this technique, a mask whose opening is shaped like a maltese cross is superimposed on a circular lens. Utilizing FIG. 2, this technique involves adding together the output of a series of these kinds of masks on an intensity basis in order to form a compound image. EQU X=.SIGMA..sub.i=1.sup.2 [Ti S.sub.i.sup.2 +S.sub.i+2 +Ti S.sub.i+4 +S.sub.i+6 ]
Although this method maintains a lateral resolution of the imaging system as determined by the main lobe beam width, there unfortunately results an increase in side lobes. Furthermore, the summation indicated above can only be achieved by sequential integration for optical systems so that the formation of the optical compound image of reduced speckle contrast is a very time consuming process.
The device of Trimmer et al. disclosed in U.S. Pat. No. 4,430,898 describes a transducer using a diamond-shaped transmit aperture and a concentric square-shaped receive mode aperture oriented at 45.degree. angle with respect to the transmit aperture in order to achieve reduced side lobes. No attempt was made in this patent to develop a compound imaging system or to develop a receive mode multiplicative processor.
Lobdell has discussed multiplicative processing for a four element array of transducers spaced 90.degree. apart in his article, "A nonlinearly processed array for enhanced azimathal resolution", IEEE Trans Son Ultrason 5U-15, 202-208, 1968. However, orthogonal multiplication was not involved nor was the formation of a compound image for speckle reduction.
Smith et al. have taught the technique of multiplicative processing in a sectored linear phased array to reduce phase aberrations from skull layers or fat layers in the article, "Signal processing techniques for improving B-mode echoencephalography, in Ultrasound in Medicine Vol. 1, Plenum Press, N.Y., 405-414, 1975. However, this technique did not involve orthogonal multiplication to reduce side lobes or the formation of a compound image.